Category: How to calculate VoIP Bandwidth

Wow this one really my hurt my head… The video training I understood, but when I applied the concept to practice questions it never worked and I was going round in circles.

This is how I was first shown how to calculate bandwidth requirements:

Payload Size (g.729 default payload = 20 bytes)

Add L3 Overhead of 40 Bytes

Add L2 overhead (Ethernet = 18, Frame Relay = 6 etc…)

Multiply by packets per second value for codec (G7.29 = 50PPS)

* 8 to go from bytes to bits

That all makes sense.. but it never worked when applied to actual examples.

So after much research, I found a great post from someone on Cisco who totally nailed it…:

*Turns out where I really going wrong was with cRTP taking the Layer 3 payload from 40 bytes to 2-4 bytes. Also the actual math approach is different here and it simply works for all real world scenarios or test questions…

Hello everyone,

I found it a little bit incorrect and too complicated to calculate the actual bandwidth used by a voice call as it is explained in the CCDA OCG:

the WAN headers size total is 7 instead of 6: the OCG forgot the byte for the end-of-frame flag; I agree that’s only a slight difference on WAN but a bigger difference on Ethernet with 4 more bytes.